Various methods have been established for reducing pollutants, in particular for reducing nitrogen oxides, in which reducing fluids (gases or liquids) are introduced into the exhaust gas system of an internal combustion engine.
SCR-technology has proved particularly advantageous for reducing nitrogen oxides, in which the nitrogen oxides (NOx) contained in the oxygen-rich exhaust gas are selectively reduced to form nitrogen and water, by means of ammonia or a precursor substance which may accordingly be converted to ammonia. Preferably, in this case aqueous urea solutions are used. The urea solution is hydrolyzed by means of hydrolysis catalysts or directly on the SCR catalyst to form ammonia and carbon dioxide. To this end, the urea solution is injected into the exhaust gas flow by means of special metering systems upstream of the hydrolysis catalyst or the SCR catalyst. In this connection, there is the problem firstly of determining the optimum quantity of reducing agent, and secondly of ensuring the correct supply and metering of the reducing agent.
If correct and reliable metering is not ensured, an efficient removal of nitrogen oxide (NOx) from the exhaust gas may not be achieved. An overdosage of reducing agent, on the other hand, may result in an undesired emission of ammonia, for example, a so-called break-through of reducing agent.
With liquid reducing agents, such as conventional urea solutions, the metering may take place by means of an injector. The activation time and thus the opening time of the injector, therefore, have an effect on the quantity of reducing agent supplied to the exhaust gas after-treatment system.
As a result of production tolerances and ageing over the lifetime of the metering system, the characteristics of the injector and thus the required opening time for the reducing agent feed may change and thus the quantity injected may not be accurately predicted. In order to achieve a maximum conversion rate of the nitrogen oxides (NOx), however, generally the metering of the reducing agent used has to be accurate and according to requirements. In order to correct inaccuracies of the injection system, it is known to use so-called closed loop controls. Said closed loop controls, however, require complicated detection in terms of measuring technology of the actual quantity of reducing agent injected.
In DE 101 00 420 A1, a method for controlling an exhaust gas aftertreatment system for an internal combustion engine is disclosed, in which a predeterminable quantity of reducing agent is supplied depending on the state of the internal combustion engine or the exhaust gas aftertreatment system. The quantity of reducing agent supplied is thus varied depending on the detection of an overdosage and/or underdosage of the reducing agent, with regard to the nitrogen oxide conversion of the SCR catalyst. The method is complicated and dependent on very many parameters, both of the internal combustion engine and the entire exhaust gas aftertreatment system. In this case, however, production tolerances, functional alterations or malfunctions of the reducing agent metering system are not taken into account.